Precoding of contention words in a fixed wireless access system

ABSTRACT

A fixed wireless access communications system comprises an access point and a plurality of subscriber units. Each subscriber unit ( 502 ) contends for access to a communications channel to the access point by transmitting a contention word to the access point during a contention time slot. The contention word is predistorted to compensate for the channel impulse response of the transmission channel so that the access point can decode the contention words without using equalisation. A control unit ( 512 ), which may comprise a microprocessor, loads the predistorted contention word (a) into a memory ( 520 ) and causes the stored predistorted contention word (a) to be applied to a transmitter ( 508 ) during a contention time slot when the subscriber unit ( 502 ) wishes to request a transmission channel to transmit data to the access point.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 application of and claims priority toInternational Patent Application Serial No. PCT/GB2003/005039, filedNov. 19, 2003, which claims priority to United Kingdom PatentApplication Serial No. 0229065.8, filed Dec. 12, 2002. The disclosuresof the prior applications are considered part of and are incorporated byreference in the disclosure of this application.

The invention relates to a fixed wireless access communications systemcomprising an access point and a plurality of subscriber units. Theinvention further relates to an access point and a subscriber unit foruse in such a system. The invention still further relates to a method ofcommunication in such a system.

Broadband wireless channels suffer from multipath propagation, whichcauses intersymbol interference (ISI) at high data rates. For FixedWireless Access (FWA) systems and wireless LAN's such as HIPERLAN, theline-of-sight (LOS) path between transmitter and receiver may beobstructed, causing severe multipath propagation, with nulls in thechannel frequency response. Typically for single-carrier systems, anequalizer is used at each end of the link to remove ISI from thereceived signal. In point-to-multipoint (PMP) networks such as FWA, anumber of terminals or subscriber units (SU's) communicate with a singlenode or access point (AP).

On the downlink, each SU only receives data from a single AP, and so itsequaliser only has to deal with a single channel impulse response.Furthermore, each SU can receive broadcast transmissions sent to allother SU's from the AP, and thus there is plenty of training data andsufficient time to train the equaliser at the SU. On the uplink, the APmust be able to receive data bursts from many SU's in quick succession,each burst arriving over a different multipath channel. The AP musttherefore rapidly re-train its equaliser for each burst from a differentSU, and this requires intensive processing, as well as large overheadsfor training symbols and training delay. “Warm start-up” avoidsre-training by storing a set of equaliser coefficients corresponding toeach SU in a look-up table at the AP. When a burst from a particular SUis scheduled to arrive at the AP, the equaliser co-efficients are loadedinto the equaliser from the look-up table, so that the data burst can bydecoded without re-training.

A difficulty arises with random-access timeslots, during which any SUmay randomly request channel access by sending an access request(contention burst) to the AP. This request mechanism is based on slottedALOHA. A contention burst received collision-free at the AP isrecognised as a valid access request, and the AP then allocates upstreamdata capacity to the requesting SU. During these random-access slots,the AP has no way of knowing beforehand which SU is sending thecontention burst, and therefore cannot use equaliser coefficients from alook-up table. Furthermore, contention bursts are typically very shortand require fast turnaround at the AP. This means that there is verylittle time available to re-train the equaliser at the AP, and onlylimited training data. It has been shown that multipath interferencegreatly degrades the efficiency of random access protocols such asALOHA.

One solution to the problem is for each SU to precode its contentionburst before transmission, so that it arrives at the AP undistorted.Unfortunately, many preceding schemes are not suitable for broadbandwireless channels which have nulls in their frequency response. Linearequalisers cannot easily deal with such channels because noiseenhancement occurs at nulls in the frequency domain and linear precodershave the same problems. Non-linear precoders avoid noise enhancement atthe nulls, but are prone to instability when the channel is notminimum-phase, and some form of limiting operation is needed to keep theprecoder stable. The drawback with non-linear precoders is the extraprocessing needed at the receiver to undo the effects of the transmitterstabilisation operation.

Precoders described in the literature typically use a filter (orcombination of filters) to invert the channel impulse response so thatany transmitted signal, after preceding and passing through the channel,is received undistorted at the receiver. The precoder transfer functionF is the inverse of the channel transfer function H, so that FH=1. Allroots of the channel impulse response (CIR) must be cancelled bycorresponding roots of F. Thus, any arbitrary data sequence x input atthe transmitter is received undistorted at the receiver. The difficultyis that CIR roots lying close to the unit circle in the z-plane aredifficult to synthesise.

In a first aspect the invention provides a fixed wireless access (FWA)communication system comprising an access point and a plurality ofsubscriber units each transmitting a predetermined data sequence:comprising means for determining the impulse response of the upstreamchannel between each subscriber unit and the access point; means forgenerating the data for transmission from a subscriber unit to theaccess point, the data comprising the predetermined data sequencepredistorted to compensate for the channel impulse response of theupstream channel between the subscriber unit and the access point; meansfor storing the predistorted predetermined data sequence within thesubscriber unit; and means for transmitting the stored sequence from thesubscriber unit to the access point when it is desired to transmit thepredetermined sequence to the access point.

In a system according to the invention, the transmitted data word ismodified so that when passed through the precoder and over a multipathchannel the original data word is received undistorted. The twooperations of modifying the data word and precoding are combined into asingle operation. In effect, this means that the precoder coefficientsare designed specifically for a unique data word to be transmitted.Although this is not very useful for the transmission of random data, ithas particular application for the upstream transmission of a contentionword from a subscriber unit to an access point. In this case, eachsubscriber unit transmits a single unique contention word that isspecific to the subscriber unit and does not change. This means that itis feasible to design a precoder which is specifically optimised for thecontention word used by each subscriber unit.

The means for determining the impulse response of the upstream channelmay comprise means for transmitting a training data sequence having goodautocorrelation properties from the subscriber unit to the access point,the training data sequence being known to the access point and means forderiving the channel impulse response from the received data sequence.All subscriber units may transmit the same training data sequence.

The access point may be arranged to transmit the respective upstreamchannel impulse response and a unique contention word to each subscriberunit, the contention word being the predetermined data sequence, themeans for generating the predistorted contention word being locatedwithin the subscriber unit.

Alternatively, the access point may be arranged to allocate a uniquecontention word to the subscriber unit; to calculate a predistortedcontention word from the upstream channel impulse response, thepredetermined contention word being such that when it is transmittedfrom the subscriber unit the access point is capable of decoding itwithout using equalisation; and to transmit to the subscriber unit thepredistorted unique contention word.

The subscriber unit may comprise an equaliser for equalising thedownstream channel between the access point and the subscriber unit: andthe determining means may comprise means for transmitting a trainingdata sequence having good autocorrelation properties from the subscriberunit to the access point; means for retransmitting the received trainingdata sequence from the access point to the subscriber unit; and meanswithin the subscriber unit for deriving the upstream channel impulseresponse from the received retransmitted training data sequence.

The means for generating the data sequence for transmission from thesubscriber unit to the access point may comprise a processor forcalculating the predistorted predetermined data sequence from theupstream channel response and the predetermined data sequence using theMoore-Penrose pseudo-inverse algorithm, or a singular-valuedecomposition (SVD) algorithm.

The invention further provides a subscriber unit for use in such a fixedwireless access system: the subscriber unit comprising means fortransmitting a training sequence having good autocorrelation propertiesover an upstream channel to the access point; means for storing inpredistorted form a unique contention word received from the accesspoint, the predistorted form being such as to compensate for the impulseresponse of the upstream channel; and means for transmitting thepredistorted contention word to the access point in order to requestaccess to a transmission channel.

The subscriber unit may be arranged to receive the contention word fromthe access point over the downstream channel together with the upstreamchannel impulse response calculated at the access point from thetraining sequence transmitted from the subscriber unit: the subscriberunit comprising means for calculating a predistorted contention wordfrom the received contention word and channel impulse response, thepredistorted contention word being such as to compensate for the impulseresponse of the upstream channel so that the access point receives thecontention word in a form which it can decode withoutusing-equalisation; and means for storing the calculated predistortedcontention word in memory.

In an alternative embodiment, the subscriber unit may be arranged toreceive a unique contention word and the training sequence retransmittedby the access point in the form received by the access point. In thiscase the subscriber unit may comprise means for determining the upstreamchannel impulse response from the received retransmitted trainingsequence; means for calculating a predistorted contention word using theunique contention word and the determined upstream channel impulseresponse such that on retransmission of the predistorted contention wordover the upstream channel the access point is able to decode thecontention word without the use of equalisation; and the means forstoring the calculated predistorted contention word in memory.

The invention still further provides an access point for use in a fixedwireless access system, the access point comprising means for receivinga training data sequence having good autocorrelation properties from asubscriber unit; means for allocating the subscriber unit a uniquecontention word; means for calculating the upstream channel impulseresponse between the subscriber unit and the access point from thereceived training data sequence; means for generating a predistortedcontention word from the calculated upstream channel impulse responseand the allocated contention word; and means for transmitting thepredistorted contention word to the subscriber unit.

The generating means may comprise a processor for generating thepredistorted contention word from the upstream channel impulse responseand the contention word using the Moore-Penrose pseudo-inversealgorithm, or a singular-value decomposition (SVD) algorithm.

Alternatively, an access point for use in the fixed wireless accesssystem may comprise means for receiving a training data sequence havinggood autocorrelation properties from a subscriber unit; means forallocating the subscriber unit a unique contention word and means fortransmitting the received training data sequence together with theunique contention word.

In a further aspect, the invention provides a method of transmitting apredetermined data sequence from a transmitter in a subscriber unit to areceiver in an access point in a fixed wireless access system over atransmission channel having a channel impulse response comprising thesteps of:

-   -   a) determining the channel impulse response    -   b) predistorting the predetermined data sequence using the        determined channel impulse response so that, when transmitted        over the channel and received at the receiver it can be decoded        without the use of equalisation; and    -   c) storing the predistorted data sequence in memory in the        subscriber unit.

Our co-pending UK Patent Application No. 0118288.0 (43199) discloses amethod of pre-distorting a predetermined data sequence to compensate forthe impulse response of a channel over which the predetermined datasequence is to be transmitted comprising the steps of;

transmitting the predetermined data sequence without precoding over thechannel using a first transmitter,

receiving the predetermined data sequence using a first receiver andequalising the received signal, using an algorithm that is constrainedto optimise the equaliser specifically for the predetermined sequence,to enable the data sequence to be decoded;

determining the equaliser coefficients required to enable the equaliserto equalise the received data sequence,

applying the determined equaliser coefficients to a second transmitter;

transmitting the equaliser coefficients to a second receiver using thesecond transmitter,

receiving the equaliser coefficients at the second receiver, and

loading the received equaliser coefficients into a precoder in the firsttransmitter when the predetermined sequence is subsequently transmittedso that it is received at the first receiver in a form suitable fordecoding without equalisation at the first receiver.

Our co-pending UK Application No. 0201738.2 (43653) discloses a methodof pre-distorting a predetermined data sequence to compensate for theimpulse response of a channel over which the predetermined data sequenceis to be transmitted comprising the steps of;

transmitting the predetermined data sequence without preceding over thechannel using a first transmitter,

receiving the predetermined data sequence using a first receiver andequalising the received signal, using an algorithm that is constrainedto optimise the equaliser specifically for the predetermined sequence,to enable the data sequence to be decoded;

determining the equaliser coefficients required to enable the equaliserto equalise the received data sequence,

applying the determined equaliser coefficients to a second transmitter;

transmitting the equaliser coefficients to a second receiver using thesecond transmitter,

receiving the equaliser coefficients at the second receiver;

forming the predistorted predetermined data sequence and storing it inmemory; and

subsequently transmitting the stored predistorted predetermined datasequence so that it is received at the first receiver in a form suitablefor decoding without equalisation at the first receiver.

In both of the above referenced applications it is necessary for thepredetermined data sequence to be transmitted without preceding todetermine the upstream channel impulse response. In contrast, in thepresent invention it is not necessary to transmit the predetermined datasequence, rather any data sequence having good autocorrelationproperties may be used provided that sequence is known to the accesspoint. In particular the same sequence may be used by all subscriberunits enabling a simplification of the access point in that it does notneed to allocate a sequence to the subscriber unit in order to enablethe upstream channel impulse response to be determined.

Step a) may comprise the steps of:

-   -   d) transmitting a training sequence from the subscriber unit to        the access point; and    -   e) calculating the channel impulse response from the received        training data sequence.

Step e) may be carried out at the access point and comprise the furthersteps of:

-   -   f) transmitting the calculated channel impulse response from the        access point to the subscriber unit; and    -   g) transmitting the predetermined data sequence from the access        point to the subscriber unit.

Steps b) and e) may be carried out in the access point and comprise thefurther step of transmitting the predistorted predetermined datasequence from the access point to the subscriber unit.

This enables a reduction in complexity of the subscriber units since thegeneration of the predistorted contention word takes place in the accesspoint. It will be apparent that in many systems there will besignificantly more subscriber units than access points and consequentlyreducing the complexity, and hence the cost, of the subscriber units canresult in a lower system cost.

The method may comprise the step of retransmitting the training datasequence from the access point to the subscriber unit wherein step e) iscarried out at the subscriber unit.

Step b) may be carried out by generating the predistorted predetermineddata sequence from the predetermined data sequence and the channelimpulse response using the Moore-Penrose pseudo-inverse algorithm, or asingular-value decomposition (SVD) algorithm.

The above and other features and advantages of the invention will beapparent from the following description, by way of example, ofembodiments of the present invention with reference to the accompanyingdrawings, in which:—

FIG. 1 shows, in block schematic form, a generalised fixed wirelessaccess communication system of known form;

FIG. 2 shows, in block schematic form, a fixed wireless access systemaccording to the invention;

FIG. 3 shows, in block schematic form, an embodiment of a subscriberunit according to the invention for use in the system of FIG. 2;

FIG. 4 shows, in block schematic form, an embodiment of an access pointaccording to the invention for use in the system of FIG. 2;

FIG. 5 shows upstream and downstream frames and shows, in particular,the contention word slots;

FIG. 6 is a flow diagram illustrating methods of setting up thecontention word for a subscriber unit;

FIG. 7 illustrates the form of the contention word at various points inits transmission from the subscriber unit to the access point; and

FIG. 8 illustrates a form of contention word.

As shown in FIG. 1 a known fixed wireless access communications networkcomprises an access point 1 and a plurality of subscriber units 2-1, 2-2to 2-N. Each subscriber unit is connected to the access point via adownstream channel h₁₋₁, h₁₋₂ and h_(1-N) and upstream channels h₂₋₁,h₂₋₂ to h_(2-N). As discussed in the introduction, each of thesechannels will suffer multipath interference and each of the channelswill have its own channel impulse response. Normally the subscriberunits will be able to hear all downstream transmissions from the accesspoint and consequently will have plenty of time to train an equaliser toremove intersymbol interference. In the upstream direction, however,each subscriber unit transmits to the access point over a separatechannel using a time division multiplex protocol. As a result, thecharacteristics of the equaliser at the access point 1 have to bechanged for each transmission from the subscriber units since thechannel impulse responses for the transmissions from differentsubscriber units will be different. During normal data transmission thismay be achieved by switching equaliser coefficients stored in a look uptable depending on which subscriber unit has been allocated theparticular upstream transmission time slot. That is the access pointknows which subscriber unit is transmitting at any particular time andcan preset its equaliser characteristics using a look up table storingthe appropriate tap co-efficients, which have been determined usingtraining sequences on previous transmissions to equalise the particularchannel impulse response in the channel between the expected subscriberunit and the access point. During a contention slot, however, the accesspoint has no knowledge of which subscriber unit is attempting tocommunicate with it. Thus, the equaliser has to be trained for everytransmission as it has no prior knowledge of which transmission channelis being used and hence what its characteristics are. This imposes alarge overhead as the subscriber unit has to transmit a trainingsequence within the contention slot in order to enable the access pointto train its equaliser.

As has been stated earlier, it is known that a precoder can be used toinvert the channel impulse response before transmission. These precodersuse some kind of filter or combination of filters to invert the channelimpulse response so that any transmitted signal, after precoding andpassing through the channel, is received undistorted at the receiver.The precoder transfer function F is the inverse of the channel transferfunction H, so that FH=1. Thus any arbitrary data sequence input at thetransmitter is received undistorted at the receiver. This holds true forall input sequences.

In our co-pending UK Patent Application No: 0106604.2 (42559) a linearprecoder is used to avoid the stability problems associated withnon-linear precoders. The linear precoder cancels all roots of thechannel impulse response except for those roots lying on the unit circlein the z plane. A root rotation method combined with pulse positionmodulation is used to remove critical zeros (zeros on the unit circle)from the channel impulse response. In effect, the input data word ismodified to cancel out those zeros of the channel impulse response whichthe precoder is unable to cancel. The difficulty with this method isthat some channels contain multiple critical zeros which are beyond thecapacity of the root rotation method to remove.

It is desirable to use pre-equalisation (preceding) for the contentionword as proposed in our co-pending application, but a more robust methodis desirable to deal with the situation where multiple critical zeros ofthe channel impulse response lie on or close to the unit circle in the zplane.

FIG. 2 shows in block schematic form a fixed wireless access system inwhich the invention may be implemented. The system shown in FIG. 2comprises an access point 501 and a plurality of subscriber units 502-1,502-2 to 502-N. Transmission between the access point 501 and thesubscriber units 502 is by means of a time division multiplex, frequencydivision duplex protocol. That is, the access point transmits data tothe subscriber units at one carrier frequency and receives transmissionsfrom each subscriber unit using a different carrier frequency, thesubscriber units all transmitting at the same carrier frequency, but intime division multiplex form. As a result, all the subscriber unitsreceive downstream data from the access point on the same carrierfrequency, albeit over different channels. That is, the downstreamchannels h₁₋₁, h₁₋₂ to h_(1-N) will have different channel impulseresponses but will continuously receive the transmissions from theaccess point even if they are not specifically addressed to thatsubscriber unit. The access point 501 will receive transmissions fromthe subscriber units over upstream channels h₂₋₁, h₂₋₂ to h_(2-N). Eachof these channels will have a different channel impulse response and thetransmissions will be time division multiplexed so that the access pointreceives bursts of data from each of the subscriber units in turn. Dueto the different carrier frequencies used for upstream and downstreamtransmission, the downstream channel impulse responses h₁₋₁ to h_(1-N)are not the same as the corresponding upstream channel responses h₂₋₁ toh_(2-N).

Data sent by the access point is not precoded as all the subscriberunits have sufficient time to equalise the channel characteristicsbetween the access point and the respective subscriber unit, since theywill normally receive all transmissions from the access point and theaccess point is transmitting relatively continuously. Thus eachsubscriber unit will include an equalizer to remove intersymbolinterference from the transmissions of the access point and thisequalizer has ample time to be trained to the channel characteristics.In the upstream direction, transmissions from the subscriber unit areprecoded before being transmitted to compensate for the impulse responseof the channel between the subscriber unit and the access point. This isbecause the access point would otherwise incur a substantial signalprocessing burden in equalising the channels from all the differentsubscriber units. This signal processing would have to be done in ashort time period otherwise the transmission overheads would becomeexcessive.

FIG. 3 shows in block schematic form an embodiment of a subscriber unit502 in which the invention may be implemented. The subscriber unit 502is connected to an antenna 506 via which communication with the accesspoint 501 is effected. Data transmitted from the access point 501 ispassed via the aerial 506 to the input of a receiver 507. The output ofthe receiver 507 is fed to the input of an equaliser 509 whose output isconnected to the input of a decoder 510. The output of the decoder 510provides a data output 511 of the subscriber unit 502. The output of thedecoder is also fed to an input of a control unit 512 which controls theoperation of the subscriber unit 502. The control unit 512 willtypically comprise a processor with associated data and programmememory. A data input 513 is connected to the input of an encoder 514which encodes the data to be transmitted to the access point 501 andassembles it into appropriate data frames under the control of thecontrol unit 512. The output of the encoder 514 is fed to the input of aprecoder 516 which precodes the data to compensate for the impulseresponse of the upstream channel to the access point 501. The precoder516 is controlled by the control unit 512 to enable it to precode thedata in such a manner as to at least partially compensate for theimpulse response of the upstream data channel. It should be noted thatthe precoder 516 is used for general data transmission and may onlypartially precode the data so that some equalisation may be necessary atthe access point for general data. The precoder 516 is not used whentransmitting the contention word. That is a predetermined data sequencethat can, according to the present invention, be sufficiently precodedat the subscriber unit to enable decoding at the access point withoutthe use of any equalisation at the access point. The output of theprecoder 516 is fed to a first input of a switching arrangement 515whose output is connected to the input of a transmitter 508. The outputof the transmitter is connected to the antenna 506 to enable the data tobe transmitted from the subscriber unit 502 to the access point 501.

The subscriber unit further comprises a memory 520 in which thepredistorted contention word is stored. The contention word is used bythe subscriber unit to request allocation of a transmission channel toenable the subscriber unit to transmit data to the access point. Thecontrol unit 512 loads a predistorted contention word into the memory520. The predistorted contention word is calculated using the contentionword allocated to the subscriber unit and the upstream channel impulseresponse.

When the subscriber unit wishes to request access to a transmissionchannel the control unit 512 causes this predistorted contention word tobe read from the memory and applied to a second input of the switchingarrangement 515. The output of the memory 520 is fed to a second inputof the switching arrangement 515 and the control unit will control theswitching arrangement 515 such that the second input from the memory 520is passed to its output to be applied to the input of transmitter 508during a contention slot when the subscriber unit wishes to request atransmission channel.

FIG. 4 shows, in block schematic form, an embodiment of an access point501 in which the invention may be implemented. An antenna 530 is coupledto the access point 501 and is connected to the input of a receiver 531.The output of the receiver 531 is connected via a switching arrangement533 having a first output coupled to the input of an equaliser 536,which may take the form of a decision feedback equaliser, and a secondoutput coupled to a bypass line 537. The output of the receiver 531 isfurther connected to an input of a correlator and channel estimator 534that establishes correct timing and provides timing information andchannel estimation to a control unit 540. The control unit 540 will,typically, comprise a microprocessor and associated programme and datamemory. The output of the equaliser 536 and bypass line 537 areconnected to the input of a decoder 538. The output of the decoder 538is fed to a data output 539 of the access point 501 and to an input ofthe control unit 540. The control unit 540 has an output which controlsthe switching arrangement 533 to select whether the bypass line 537 orequaliser 536 are used in the decoding of the data sent from thesubscriber units. The control unit 540 also enters data into a look-uptable 541, which may take the form of a read-write memory, and controlsthe reading out of the data into the equaliser 536 to enable theappropriate equaliser taps to be set depending on which subscriber unitis transmitting to the access point at the time. An input 542 receivesdata for transmission and is connected to the input of an encoder 543whose output is fed to the input of a transmitter 532. The encoder 543is controlled by an output from the control unit 540 which causes thedata to be encoded and assembled into the appropriate frame structure.

FIG. 5 shows downstream (access point to subscriber unit) and upstream(subscriber unit to access point) transmission frames. The downstreamframes 200 comprise an initial synchronisation sequence 202 and datafields 203. Contention allocation data fields 201-1 to 201-3 are alsotransmitted to identify to individual subscriber units when they areallocated data slots to transmit data to the access point over theupstream channels. Clearly the number of contention allocation datafields in a frame may be greater or less than three.

The upstream frames 210 contains contention slots 211-1 to 211-3 duringwhich a subscriber unit can transmit a contention word to the accesspoint in order to request transmission time. The upstream frame alsoincludes data fields 213 that are allocated by the access point toparticular subscriber units to transmit general data to the access pointon the basis of contention requests made by the subscriber units.

It will be apparent to the skilled person that during the data fields213 the access point will know which subscriber unit is transmittingsince the access point allocated individual data fields 213 to selectedsubscriber units. As a result, the access point has a knowledge of thechannel impulse response and can use an equaliser having stored presetcharacteristics to aid in decoding the data. During the contention slots211-1 to 211-3, however, the access point does not know the channelcharacteristic, since it does not know until it has decoded thecontention word which subscriber unit is transmitting. Accordingly, in asystem according to the invention the contention word is fully precodedso that equalisation is not needed to decode the contention word at theaccess point but the general data may be only partially precoded withsome equalisation taking place at the access point since, for generaldata, the access point has some knowledge of the upstream channelimpulse response.

FIG. 6 is a flow-diagram showing how the contention word is set up in asystem as shown in FIG. 2. The first stages of the registration process,block 601, may follow the procedure described in our co-pending UKpatent applications numbers 0113887.4 (42557) and 0113888.2 (43066) orother techniques known in the art. In order for a subscriber unit torequest upstream transmission time, it must first contend for channelaccess using a contention slot. The final stage of the subscriber unitregistration involves setting up the contention process for thatparticular subscriber unit as illustrated in the flow diagram of FIG. 6.After the first registration steps of power control initialisation anddown stream equaliser setting as, for example, shown in the two patentapplications referenced above, the access point transmits a uniquesubscriber unit identifier SU-ID to the subscriber unit to be used toidentify that subscriber unit in all future communications. In ourco-pending UK patent application number 0113888.2 (43066) a procedure isdisclosed in which the contention word is calculated by using a testcontention word sent from the subscriber unit to the access point, thetest contention word having been previously allocated to the subscriberunit by the access point. In a system according to the presentinvention, an estimate of the uplink channel is required rather than atest contention word sent from the subscriber unit. In the case of atime division duplex system the subscriber unit may simply make achannel estimate based on downstream transmission from the access point.In the case of a frequency division duplex system, however, in which theupstream and downstream channels cannot be assumed to be identical theaccess point must make a channel estimate based on the data receivedfrom the subscriber unit. This is based on a known training sequencesent without precoding by the subscriber unit in a similar way to thetest contention word in UK patent application number 0113888.2 (42066).This training sequence need not, however, in this case be the same asthe contention word. It may simply be any data sequence with goodautocorrelation properties that is known to both the subscriber unit andthe access point.

FIG. 6 illustrates three possible methods of generating a predistortedcontention word. In the first method the first step is to estimate theupstream channel impulse response, illustrated by box 602. A method bywhich this channel estimation may be achieved will be explained later.Having estimated the upstream channel impulse response the access point501 uses the upstream channel impulse response and the contention wordthat it is allocating to the particular subscriber unit 502 to calculatea predistorted contention word. This is illustrated in step 603. Havingcalculated the predistorted contention word the access point 501 sends asubscriber unit identifier to the subscriber unit 502 together with thepredistorted contention word, illustrated by step 604. When thesubscriber unit 502 receives the predistorted contention word from theaccess point 501 it stores the received predistorted contention word inthe memory 520. This is illustrated in step 605. The subscriber unit 502identifies that the predistorted contention word is allocated to itselfby means of the associated subscriber unit identifier. The final step606 is to acknowledge that the contention word has been received andthat the contention is now set up.

In a second method the access point 501 again estimates the upstreamchannel impulse response in step 602, but then the access point in step610 sends the subscriber unit identifier, the contention word, and thechannel impulse response to the subscriber unit. Then, in step 611, thesubscriber unit uses the contention word and upstream channel impulseresponse to calculate a predistorted contention word. The subscriberunit then follows the procedure of the first method and stores thepredistorted contention word in the memory 520.

In the third method the access point 501, instead of estimating theupstream channel impulse response, retransmits samples of the trainingsequence that it received to the subscriber unit 502 together with thesubscriber unit identifier and the contention word that the access point501 has allocated to that particular subscriber unit 502. This isillustrated in step 620. Step 621 is the subscriber unit estimating theupstream channel impulse response from the retransmitted samples of thereceived training sequence. The next step 622 is for the subscriber unitto calculate the upstream channel impulse response from the trainingsequence that has been retransmitted by the access point and topredistort the contention word the access point has allocated to it. Thethird method then enters step 605 and the predistorted contention wordis now stored in the memory 520 and the contention word set upacknowledged in step 606 in the same manner as the first and secondmethods.

In order to calculate the predistorted contention word the uplinkchannel h=[h₁, h₂ . . . h_(L)]^(T) is first calculated. This is step 602in the first and second methods and step 621 in the third method. Onemethod is to use least-squares in the form of the Moore-Penrosepseudo-inverse (MPPI) algorithm to solve the equation: Xh=y where y isthe sequence of received training sequence samples after transmissionover the channel, and X is a matrix constructed from the known trainingsequence as follows:

$X = \begin{bmatrix}x_{N} & x_{N - 1} & \ldots & x_{1} \\x_{N + 1} & x_{N} & \ldots & x_{2} \\\vdots & \vdots & \; & \vdots \\x_{{2N} - 1} & x_{{2N} - 2} & \ldots & x_{N}\end{bmatrix}$

The MPPI algorithm may be used to calculate h as: h=X^(−#)y where:X^(−#)=(X^(*T)X)⁻¹.X^(*T)

The channel estimate h may also be obtained using recursive algorithmssuch as RLS or LMS, or else simply using a correlator acting on samplesof the received training sequence. However, the least-squares solutioncalculated using the MPPI equation, is the presently preferred method.

Once the uplink channel estimate h has been obtained, the MPPI algorithmis used to solve for the best predistorted sequence a to transmit at thesubscriber unit in order to receive the undistorted contention word c atthe access point. This is step 603 in the first method, step 611 in thesecond method, and step 622 in the third method. This predistorted(precoded) contention word a, which when passed through the channelproduces the original contention word c at the receiver, is calculatedin the following manner:c=Ha  (1)where

c=[c₁, c₂, . . . , C_(K)]^(T)=the original contention word

H is the upper triangular channel matrix, of dimension K×N:

$H = \begin{bmatrix}h_{L} & h_{L - 1} & \ldots & h_{1} & 0 & 0 & \ldots & 0 \\0 & h_{L} & \ldots & h_{2} & h_{1} & 0 & \ldots & 0 \\\vdots & \vdots & \; & \vdots & \vdots & \vdots & \; & \vdots \\0 & 0 & \ldots & 0 & 0 & h_{L} & \ldots & h_{1}\end{bmatrix}$

A cyclic prefix at least as long as the CIR is added to α to form a oflength N=(M+L):a=[α _(M−L+1), α_(M−L+2), . . . , α_(M), α₁, α₂, . . . α_(M)]^(T)  (2)

L<K<M

Equation (1) can be re-written in terms of a instead of a:

$\begin{matrix}{{c = {\Omega\alpha}}{{where}\text{:}}{\Omega = \begin{bmatrix}h_{1} & 0 & \ldots & 0 & 0 & h_{L} & h_{L - 1} & \ldots & h_{3} & h_{2} \\h_{2} & h_{1} & \ldots & 0 & 0 & 0 & h_{L} & \ldots & h_{4} & h_{3} \\\vdots & \vdots & \; & \vdots & \vdots & \vdots & \vdots & \; & \vdots & \vdots \\0 & 0 & \ldots & h_{L} & h_{L - 1} & h_{L - 2} & h_{L - 3} & \ldots & h_{1} & 0 \\0 & 0 & \ldots & 0 & h_{L} & h_{L - 1} & h_{L - 2} & \ldots & h_{2} & h_{1}\end{bmatrix}}{\alpha = {{\Omega^{- \#}c} = {\left( {\Omega^{*T}\Omega} \right)^{- 1}\Omega^{*T}c}}}} & (3)\end{matrix}$where Ω^(−#) is the Moore-Penrose pseudo-inverse (MPPI) of Ω. The kernelof the precoded contention word α is then cyclically extended to formthe full precoded contention word a before transmission in an accessrequest timeslot.

Since Ω is a circulant matrix, an alternative method (which offersbetter numerical properties) for calculating α is to use singular-valuedecomposition (SVD) as follows:Ω=Q ₁ ΛQ ₂ ^(*T)(singular value decomposition of Ω), and so the pseudo-inverse iscalculated as:Ω^(−#) =Q ₂Λ^(−#) Q ₁ ^(*T)The matrices Q₁ and Q₂ are unitary matrices, and Λ is a diagonal matrixas follows: Λ=diag[λ₁, . . . , λ_(N)] and its inverse is: Λ^(−#)=diag[λ₁⁻¹, . . . , λ_(N) ⁻¹].

FIG. 7 shows the form of the contention word a as transmitted by thesubscriber unit SU and applied to the upstream channel h, the receivedsequence at the access point AP after transmission through the upstreamchannel h, and the decoded contention word c at the output of the accesspoint AP.

FIG. 8 shows the form of the contention word used in the presentembodiment. It comprises a synchronisation field sync, a uniqueidentifier SU-ID for the particular subscriber unit, and a cyclicredundancy code CRC for error detection and/or correction. Clearly, theform of the contention word could be different from that shown in FIG.8, which is merely an example of a suitable contention word and manymore other forms would be apparent to the skilled person.

1. A fixed wireless access (FWA) communications system comprising anaccess point and a plurality of subscriber units each transmitting apredetermined data sequence: comprising means for determining theimpulse response of the upstream channel between each subscriber unitand the access point; means for generating a data sequence fortransmission from a subscriber unit to the access point, the datacomprising the predetermined data sequence pre-distorted to compensatefor the channel impulse response of the upstream channel between thesubscriber unit and the access point, means for storing thepre-distorted predetermined data sequence within the subscriber unit,and means for transmitting the stored sequence from the subscriber unitto the access point when it is desired to transmit the predeterminedsequence to the access point; wherein the means for generating the datasequence for transmission from the subscriber unit to the access pointcomprises a processor for calculating the pre-distorted predetermineddata sequence from the upstream channel response and the predetermineddata sequence using the Moore-Penrose Pseudo-Inverse Algorithm.
 2. Asystem as claimed in claim 1 in which the means for determining theimpulse response of the upstream channel comprises means fortransmitting a training data sequence having good auto correlationproperties from the subscriber unit to the access point, the trainingdata sequence being known to the access point, and means for derivingthe channel impulse response from the received data sequence.
 3. Asystem as claimed in claim 2 in which all subscriber units transmit thesame training data sequence to enable their respective upstream channelimpulse responses to be determined.
 4. A system as claimed in claim 1 inwhich the access point is arranged to transmit the respective upstreamchannel impulse response and a unique contention word to each subscriberunit, the contention word being the predetermined data sequence, themeans for generating the pre-distorted contention word being locatedwithin the subscriber unit.
 5. A system as claimed in claim 2 in whichthe access point is arranged to transmit the respective upstream channelimpulse response and a unique contention word to each subscriber unit,the contention word being the predetermined data sequence, the means forgenerating the pre-distorted contention word being located within thesubscriber unit.
 6. A system as claimed in claim 3 in which the accesspoint is arranged to transmit the respective upstream channel impulseresponse and a unique contention word to each subscriber unit, thecontention word being the predetermined data sequence, the means forgenerating the pre-distorted contention word being located within thesubscriber unit.
 7. A system as claimed in claim 1 in which the accesspoint is arranged to allocate a unique contention word to the subscriberunit, to calculate a pre-distorted contention word from the upstreamchannel impulse response, the pre-distorted contention word being suchthat when it is transmitted from the subscriber unit the access point iscapable of decoding it without using equalisation, and to transmit tothe subscriber unit the pre-distorted unique contention word.
 8. Asystem as claimed in claim 2 in which the access point is arranged toallocate a unique contention word to the subscriber unit, to calculate apre-distorted contention word from the upstream channel impulseresponse, the pre-distorted contention word being such that when it istransmitted from the subscriber unit the access point is capable ofdecoding it without using equalisation, and to transmit to thesubscriber unit the pre-distorted unique contention word.
 9. A system asclaimed in claim 3 in which the access point is arranged to allocate aunique contention word to the subscriber unit, to calculate apre-distorted contention word from the upstream channel impulseresponse, the pre-distorted contention word being such that when it istransmitted from the subscriber unit the access point is capable ofdecoding it without using equalisation, and to transmit to thesubscriber unit the pre-distorted unique contention word.
 10. A systemas claimed in claim 1 in which the subscriber unit comprises anequaliser for equalising the downstream channel between the access pointand the subscriber unit, and the determining means comprises means fortransmitting a training data sequence having good auto correlationproperties from the subscriber unit to the access point, means forre-transmitting the received training data sequence from the accesspoint to the subscriber unit, and means within the subscriber unit forderiving the upstream channel impulse response from the receivedre-transmitted training data sequence.
 11. A system as claimed in claim10 in which the predetermined data sequence is a contention word whichis unique to each subscriber unit in the system.
 12. A subscriber unitfor use in a fixed wireless access system as claimed in claim 1: thesubscriber unit comprising means for transmitting a training sequencehaving good auto correlation properties over an upstream channel to theaccess point; means for storing, in pre-distorted form, a uniquecontention word received from the access point, the pre-distorted formbeing such as to compensate for the impulse response of the upstreamchannel; and means for transmitting the pre-distorted contention word tothe access point in order to request access to a transmission channel.13. A subscriber unit as claimed in claim 2 arranged to receive thepre-distorted contention word from the access point over the downstreamchannel.
 14. A subscriber unit as claimed in claim 12 arranged toreceive the contention word from the access point over the downstreamchannel together with the upstream channel impulse response calculatedat the access point from the training sequence transmitted by thesubscriber unit, wherein the subscriber unit comprises means forcalculating a pre-distorted contention word from the received contentionword and channel impulse response , the pre-distorted contention wordbeing such as to compensate for the impulse response of the upstreamchannel so that the access point receives the contention word in a formwhich it can decode without using equalisation, and means for storingthe calculated pre-distorted contention word in memory.
 15. A subscriberunit as claimed in claim 14 in which the calculating means comprises aprocessor for calculating the upstream channel response from thereceived re-transmitted training data sequence.
 16. A subscriber unit asclaimed in claim 12 arranged to receive a unique contention word and thetraining sequence re-transmitted by the access point in the formreceived by the access point: the subscriber unit comprising means fordetermining the upstream channel impulse response from the receivedre-transmitted training sequence; means for calculating a pre-distortedcontention word using the unique contention word and the determinedupstream channel impulse response such that on transmission of thepre-distorted contention word over the upstream channel the access pointis able to decode the contention word without the use of equalisation;and means for storing the calculated pre-distorted contention word inmemory.
 17. A subscriber unit as claimed in claim 14 in which the meansfor calculating the pre-distorted contention word comprises a processorusing the Moore-Penrose Pseudo-Inverse Algorithm to calculate thepre-distorted contention word from the upstream channel impulse responseand the received contention word.
 18. A subscriber unit as claimed inclaim 15 in which the means for calculating the pre-distorted contentionword comprises a processor using the Moore-Penrose Pseudo-Image InverseAlgorithm to calculate the pre-distorted contention word from theupstream channel impulse response and the received contention word. 19.A subscriber unit as claimed in claim 16 in which the means forcalculating the pre-distorted contention word comprises a processorusing the Moore-Penrose Pseudo-Inverse Algorithm to calculate thepre-distorted contention word from the upstream channel impulse responseand the received contention word.
 20. A subscriber unit as claimed inclaim 17 in which the means for calculating the pre-distorted contentionword comprises a processor using the Moore-Penrose Pseudo-InverseAlgorithm to calculate the pre-distorted contention word from theupstream channel impulse response and the received contention word. 21.An access point for use in a fixed wireless access system as claimed inclaim 1: the access point comprising means for receiving a training datasequence having good auto correlation properties from a subscriber unit;means for allocating the subscriber u nit a unique contention word;means for calculating the upstream channel impulse response between thesubscriber unit and the access point from the received training datasequence; means for generating a pre-distorted contention word from thecalculated upstream channel impulse response and the allocatedcontention word; and means for transmitting the pre-distorted contentionword to the subscriber unit.
 22. An access point as claimed in claim 21in which the generating means comprises a processor for generating thepre-distorted contention word from the upstream channel impulse responseand the contention word using the Moore-Penrose Pseudo-InverseAlgorithm.
 23. An access point for use in a fixed wireless access systemas claimed in claim 1 comprising means for receiving a training datasequence having good auto correlation properties from a subscriber unit;means for allocating the subscriber unit a unique contention word; andmeans for transmitting the received training data sequence together withthe unique contention word.
 24. A method of transmitting a predetermineddata sequence from a transmitter in a subscriber unit to a receiver inan access point in a fixed wireless access system over a transmissionchannel having a channel impulse response comprising the steps of: a)determining the channel impulse response, b) pre-distorting thepredetermined data sequence using the determined channel impulseresponse so that when transmitted over the channel and received at thereceiver it can be decoded without the use of equalisation, bygenerating the pre-distorted predetermined data sequence from thepredetermined data sequence and the channel impulse response using theMoore-Penrose Pseudo-Inverse Algorithm, and c) storing the pre-distorteddata sequence in memory in the subscriber unit.
 25. A method as claimedin claim 24 in which step a) comprises the steps of; d) transmitting atraining data sequence from the subscriber unit to the access point and,e) calculating the channel impulse response from the received trainingdata sequence.
 26. A method as claimed in claim 25 in which step e) iscarried out at the access point and comprising the further steps of; f)transmitting the calculated channel impulse response from the accesspoint to the subscriber unit, and g) transmitting the predetermined datasequence from the access point to the subscriber unit.
 27. A method asclaimed in claim 25 in which steps b) and e) are carried out in theaccess point and comprising the further step of transmitting thepre-distorted predetermined data sequence from the access point to thesubscriber unit.
 28. A method as claimed in claim 25 comprising the stepof re-transmitting the training data sequence from the access point tothe subscriber unit wherein step e) is carried out at the subscriberunit.